Variable attenuator

Abstract

A variable attenuator (v-ATT) is disclosed. The v-ATT includes an input terminal, an output terminal, a transmission line between the input and output terminals, at least two stages provided between the transmission line and the ground, and a bias unit. Each of the stages includes a field effect transistor (FET) that varies impedance between the transmission line and the ground according to a bias provided to the gate thereof. The bias unit generates the biases each provided to the stages. One of the features of the v-ATT is that at least one of the stages receives at least one of the biases that is different from biases provided to other of the at least one of the stages.

What is claimed is: 
     
         1 . A variable attenuator, comprising:
 an input terminal that receives a signal containing high frequency components;   an output terminal that outputs another signal attenuated from the signal received at the input terminal;   a transmission line that couples the input terminal with the output terminal, the transmission line;   a first stage connected between the transmission line and a ground, the first stage including a first field effect transistor (FET) having a gate, a current terminal connected with the transmission line, and another current terminal connected to the ground, the FET varying impedance between the current terminal and the another current terminal according to a first bias supplied to the gate;   a second stage connected between the transmission line and the ground, the second stage including a second FET having gate, a current terminal connected with the transmission line, and another current terminal connected to the ground, the second FET varying impedance between the current terminal and the another current terminal thereof according to a second bias supplied to the gate thereof;   a bias unit including an input, the bias unit generating the first bias and the second bias according to a control signal provided to the input of the bias unit, the bias unit supplying the first bias to the first stage and the second bias to the second stage,   wherein the first bias is different from and independent of the second bias.   
     
     
         2 . The variable attenuator according to  claim 1 ,
 wherein the first bias has a range and a center of the range each different from a range and a center of the range of the second bias.   
     
     
         3 . The variable attenuator according to  claim 1 ,
 further including another stage connected between the transmission line and the ground, the another stage including another FET having a gate, a current terminal connected with the transmission line, and another current terminal connected with the ground, the another FET varying impedance between the current terminal and the another current terminal of the another FET according to another bias supplied to the gate thereof,   wherein at least the first bias supplied to the first stage is different from the second bias and the another bias.   
     
     
         4 . The variable attenuator according to  claim 3 ,
 wherein the first stage is arranged closest to the input terminal.   
     
     
         5 . The variable attenuator according to  claim 1 ,
 wherein the first stage is arranged closer to the input terminal, and includes an additional FET, the first FET and the additional FET constituting a cascade connection between the transmission line and the ground, and   wherein the first FET and the additional FET receive the first bias in respective gates thereof.   
     
     
         6 . The variable attenuator according to  claim 5 ,
 wherein the second stage is arranged closer to the output terminal and includes no additional FETs.   
     
     
         7 . The variable attenuator according to  claim 1 ,
 wherein the bias unit includes a first resistive divider having a first dividing ratio and a second resistive divider having a second dividing ratio different from the first dividing ratio, the first bias being formed by the first resistive divider by dividing the control signal according to the first dividing ratio, the second bias being formed by the second resistive divider by dividing the control signal according to the second dividing ratio.   
     
     
         8 . The variable attenuator according to  claim 7 ,
 wherein at least the first resistive divider includes a diode that shifts the first bias by a forward voltage of the diode.